This invention relates to the separation of chrominance and luminance frequency components of a composite video signal, and particularly to a multi-standard adaptive chrominance separator which responds to horizontal high frequency luminance detail as well as to vertical chroma detail.
In the course of recovering video signals previously recorded on, for example, a magnetic video tape, various signal processing functions such as color processing and dropout correction are employed to insure the integrity of the video signal. Further, in sophisticated videotape recorders, color pictures are provided at non-standard speeds such as, for example, shuttle, stop and slow motion speeds, which requires additional video processing procedures such as video noise reducing. These various color video signal processes generally are performed in circuitry conventionally known as an output processor, located within a time base corrector (TBC) device. In order to perform these, and other, processes on a color video signal, it first is necessary to separate the chrominance and luminance frequency components of the composite color video signal recovered from the videotape, whereby the chrominance frequency component may be variously manipulated separately from the luminance frequency component.
The periodic nature of television scanning and the frequency-interleaving formats of a television picture (as in the NTSC and PAL color television standards) together produce a signal having a spectral distribution, wherein signal energy appears in discrete bands separated by energy nulls at intervals of line-scanning frequency. At higher frequencies, luminance and chrominance appear interleaved, separated by one-half line frequency. Accordingly, in order to provide the high quality signal processing required in recovering color video signals from videotape at non-standard speeds, precise separation of these luminance and chrominance frequency components in the frequency region of, for example, 3.58 MHz in the NTSC color television standard, is essential.
To this end, comb filters with their pass-band and stop-band characteristics are very effective in passing or rejecting one or the other of the luminance and chrominance frequency components which make up the composite video signal. That is, comb filters are more efficient in separating the components than are bandpass filters since the latter also separate out any high frequency signals other than chrominance, for example, luminance frequency component signals, which may fall in the chrominance passband of 3.58 MHz (NTSC) subcarrier frequency. Since the luminance component has no subcarrier it tends to be similar on a number of successive lines. Thus, adding the values of three successive horizontal scan lines (termed "top", "middle" and "bottom" data lines) using a comb filter with a weighting factor of one-half the middle line and minus one-quarter of the top and bottom lines, provides a luminance frequency component value of nearly zero. It follows that a comb filter will provide a chrominance frequency component signal with less luminance frequency component remaining than will a bandpass filter.
However, a comb filter has the inherent disadvantage of averaging video from several adjacent lines together. In a situation where a sudden color change occurs from one scan line to the next one or two lines, the comb filter averages the vertical color transition resulting in a blurred or "soft" color transition which is sufficiently visible to be objectionable.
In the area of chrominance and luminance separation of an NTSC-encoded color television signal, there is available a scheme for detecting the occurrence of vertical chrominance transitions, and for making a decision of whether to employ a comb filter to perform separation of luminance and chrominance, or whether to maintain the luminance component separation process via use of a lowpass filter. Such a scheme is typical of the latest techniques presently available in the art of chroma/luminance separation, and thus is discussed here. The scheme detects any difference in chrominance between the top and bottom lines of three adjacent lines, and compares the difference indicative of vertical chrominance change to a fixed reference corresponding to a preselected, manually adjusted, threshold level. A gating signal is generated in the situations where sufficient vertical chrominance change exists between lines which, in turn, causes the signal to be filtered by the lowpass filter to remove spurious chrominance frequency components. If there is little or no vertical chrominance difference, the scheme passes the wideband delayed luminance signal.
The schemes of bypassing a comb filter when vertical chrominance detail is detected have the disadvantage that they may erroneously bypass the comb filter in situations where it is preferable not to, because horizontal luminance detail is not considered when making the bypass decision. As generally known and discussed above, the horizontal luminance component is best separated by a comb filter and accordingly there are situations where the detection of vertical chrominance detail should not be used to determine that the comb filter be bypassed. For example, high frequency luminance components which fall in the chrominance signal passband generate a situation where a comb filter should be employed. However, in the above type of scheme the decision to bypass the comb filter is made erroneously based only on the vertical chrominance component level. This allows the horizontal high frequency luminance to pass along with the chrominance signal, whereupon the luminance is modified as though it were chrominance in subsequent color signal processing circuits, resulting in the generation of corresponding undesirable artifacts in the picture. In addition, the vertical chrominance component level is compared against a fixed voltage level reference arrived at subjectively. Thus, in such a scheme, a change in the video input signal level causes a corresponding, and erroneous, change in the fixed level comparison of the vertical chrominance component.
Further, presently available chroma separators have the disadvantage of switching between the conventional bandpass filter output and the comb filter output; i.e., essentially switch the comb filter into, and out of, the luminance/chrominance separating system to provide the chrominance output signal. Since there can be a significant difference between the output signals generated by the bandpass filter and the comb filter, abruptly bypassing the comb filter may produce a disturbance in the picture.
Still another disadvantage of present chroma separators is their inability to be readily adapted to a switchable implementation for use with the present color television standards such as NTSC and PAL, particularly in a common piece of apparatus.
The invention overcomes the disadvantages of present chroma separators and provides a digital multi-standard system which makes a comb filter bypass decision based not only on the vertical chrominance detail, but also on the horizontal high frequency luminance detail in the television picture. In essence, the bypass decision is based on the more efficient use of the relative levels of both the chrominance and luminance detail in the picture. More particularly, the invention compares the vertical chrominance signal level against the horizontal high frequency luminance signal level, not against a fixed voltage level reference, and makes an adaptive decision based on the relative signal levels. The adaptive decision is made on a sample-by-sample basis. As a further advantage the adaptive decision is not affected by changes in the video input signal level.
In addition, the invention provides not only bandpass filter and comb filter outputs, but further provides an average chrominance signal output which is the average of the bandpass and comb filter signals. Thus in situations where there is a significant difference between the chrominance and luminance levels on adjacent lines, the adaptive decision steps through the average output when switching from bandpass to comb filter outputs, and vice versa. This provides a more gradual change in chrominance signal output and minimizes the generation of any corresponding disturbance in the picture, that is, reduces any disturbance by one-half.
Still further, the present digital adaptive chroma separator provides the highly desirable advantage of readily being adaptable to use with all present color television standards and thus contemplates a single piece of apparatus which accommodates the NTSC, PAL and SECAM standards.
To this end, the invention includes a chrominance comb filter for receiving three adjacent lines of video from a bandpass filter, and for supplying the usual bandpass chrominance signal and a comb chrominance signal to respective positions of an adaptive switch. The bandpass and comb chrominance signals also are coupled to an adder which supplies an average chrominance signal formed of one-half the sum of the bandpass and comb filter chrominance signals. This average chrominance signal also is coupled to the adaptive switch. In addition, the bandpass and comb filters are coupled to an adaptive switch controller means formed of a vertical chrominance detector channel and a horizontal high frequency luminance detector channel. The detector channels are, in turn, coupled to a comparison function means which generates an adaptive switch control signal in response to the relative comparison of the vertical chrominance level and the horizontal high frequency luminance level. The switch control signal is, for example, a 2-bit word which enables one of the three positions of the adaptive switch, to provide accordingly one of the three respective chrominance output signals.